1. Field of the Invention
The present invention relates to a method of manufacturing an optical component such as a lens and an apparatus for manufacturing the optical component.
2. Description of the Related Art
Optical components made of resin are usually manufactured by injection molding, press molding, or a similar method.
In injection molding, a molten plasticized resin material is filled in a cavity of a mold and is then cooled, whereby a molded product with a desired shape is obtained. Since the resin material is cooled by the transfer of heat from the resin material to the mold, a surface portion of the resin material that is in contact with the mold solidifies initially and an inner portion thereof then solidifies gradually. In this course, cooling shrinkage occurs; hence, a surface portion of the molded product that has solidified initially is high in density and an inner portion thereof is low in density.
When the molded product is an optical component for transmitting light like a lens, there is a problem in that the molded product has reduced optical performance due to the internal density distribution and birefringence of the molded product that are caused by shrinkage. Furthermore, there is a problem in that the molding time is long because the resin material is entirely melted and cooling takes a long time.
In press molding, a plate or a substrate having a shape similar to a final desired shape is prepared. The plate or substrate is placed into a mold heated to a temperature sufficient to mold the substrate, and is then pressed, whereby the shape of the mold is transferred to the substrate. The substrate is cooled by cooling the mold. Therefore, there is a problem in that the molding time is long because press molding takes a longer time for heating and cooling than injection molding.
In order to solve these problems, machines and a method described below have been proposed. Japanese Patent Laid-Open No. 2-95813 (hereinafter referred to as Patent Document 1) discloses a press molding machine including a substrate transmitting no ultraviolet light, a mold transmitting ultraviolet light, and an ultraviolet irradiator placed behind the mold. The substrate is heated in such a manner that the substrate is irradiated with ultraviolet light through the mold. After a surface portion of the substrate is melted, the substrate is pressed against the mold, whereby transfer is performed.
Japanese Patent No. 3169786 (hereinafter referred to as Patent Document 2) discloses an injection molding machine that includes a mold having a cavity and a non-transferable zone made of an infrared transmitting material and also includes a light source emitting infrared light. The non-transferable zone is irradiated with infrared light during injection, whereby the viscosity of resin placed on non-transferable zone is reduced.
Japanese Patent Laid-Open No. 2001-158044 (hereinafter referred to as Patent Document 3) discloses a molding method in which a transfer surface of a substrate is irradiated with infrared light through a stamp made of an infrared transmitting material in such a state that the stamp is in tight contact with the substrate. Since resin located near the transfer surface absorbs infrared light, the temperature thereof increases and therefore the viscosity thereof reduces. However, the stamp transmits infrared light and therefore the temperature thereof hardly increases.
In each of Patent Documents 1 to 3, a surface of the substrate is heated by making use of the fact that the mold transmits ultraviolet or infrared light, which is absorbed by resin, and the mold, which is made of the ultraviolet or infrared light transmitting material, and the substrate, which is made resin, has extremely lower thermal conductivity as compared with metals. The heating of the substrate surface reduces the viscosity of resin located near the transfer surface to increase the transferability thereof. The temperature of the mold can be kept low and therefore the molding time can be reduced.
The techniques disclosed in Patent Documents 1 to 3 have the effect of increasing the transferability of a substrate by heating a surface of the substrate with infrared or ultraviolet light. Since sufficient transferability is achieved even though the temperature of a mold is kept low, these techniques also have the effect of reducing the time taken for cooling, that is, the molding time.
Infrared transmitting materials such as quartz have higher thermal conductivity as compared with common resins. Therefore, in the case of using the technique disclosed in Patent Document 2 or 3, most of the heat of a surface portion of resin heated with infrared light is dissipated through a mold because the resin is in contact with the mold.
Conventional techniques have the effect of increasing the transferability of substrates by heating surfaces of the substrates and are, however, incapable of intensively heating the substrate surfaces. That is, the substrates are entirely melted by heating. Therefore, density distribution and birefringence are caused in molded products during cooling and solidification, leading to a problem with a reduction in optical performance. Furthermore, the use of ultraviolet light as a light source deteriorates resin to cause a reduction in optical performance.